Commit 1a9fba804d made an incorrect simplification. It observed that
thief robots are not boss robots and vice versa, and from that changed:
```
if (is_thief) { drop_stolen_items(); }
if (is_boss) { start_boss_death_sequence(); }
else if (death_roll) { start_robot_death_sequence(); }
else { regular_death(); }
```
to
```
if (is_thief) { drop_stolen_items(); }
else if (is_boss) { start_boss_death_sequence(); }
else if (death_roll) { start_robot_death_sequence(); }
else { regular_death(); }
```
This was incorrect, because although a thief is not a boss, a thief does
need to proceed through the other non-boss if tests. This error caused
a thief not to explode on death, and instead continue to fly around and
absorb weapon fire.
Fix the logic error by removing the incorrect `else`, so that a thief
can be tested for is_boss, get false, and then proceed through the
non-boss death logic.
Fixes: 1a9fba804d ("Avoid repeated valptridx dereferences in multibot.cpp")
0de494c2e0